This research is broadly concerned with interactions between microbial enzymes and the secretory immune system in man. Secretory immunity is mediated largely by antibodies of the IgA class and we have found in earlier work that microorganisms responsible for human disease produce an extracellular enzyme that is capable of proteolytically cleaving IgA to presumably render it inactive. Our research involves the cultivation of these bacteria, the isolation and characterization of the IgA protease enzymes produced by them, and the characterization of the fragments of immunoglobulin A resulting from their action. The research is broadly concerned with understanding how, if at all, these bacteria serve the organisms in colonizing the mucosal tissues and in causing inflammatory and infectious illness.